Betaines are used on a large scale for the preparation of soaps and detergents for the body and particularly for the preparation of hair shampoos. Efforts are therefore made to prepare betaines which are free of impurities that can cause skin irritations or are otherwise undesirable for physiological reasons.
The German patent 29 26 479 discloses a method of the abovedescribed type, which is characterized in that the quaternizing reaction is carried out in an alkaline solution, which has a pH of 7.5 to 10.5 measured at 98.degree. C.
By maintaining the pH within this range during the quaternization reaction, the fatty acid amide dialkylamine can no longer be detected by thin-layer chromatography after a reaction time of about three hours, the limit of detection being at about 0.02% by weight. If the reaction time is increased to about 8 to 10 hours, the content of organically bound chlorine in the reaction product is also decreased. However, by refining the analytical methods, it has been possible to show that residual amounts of compounds with organically bound chlorine, particularly sodium dichloroacetate, are still contained in the betaine obtained even under these conditions. The use of a pH higher than 10.5 also leads only to an inadequate decrease in the residual amounts of chloroacetic acids. Moreover, excessively long reaction times would be required for this, which are prohibitive for economic reasons. Furthermore, the danger exists of increasingly decomposing the starting materials or the betaines formed as a result of, for example, the hydrolysis of the amide bond. Above a pH of 10.5, the decomposition of the product can also be expected.
The German Offenlegungsschrift 39 39 264 relates to a method for lowering the residual content of free alkylating agent in aqueous solutions of amphoteric or zwitterionic surfactants, with the distinguishing feature that the solutions are given an aftertreatment with ammonia, an amino acid with 2 to 8 carbon atoms or an oligopeptide. The residual content of free alkylating agent, particularly of chloroacetic acid, is to be reduced to values of less than 0.01% by weight (based on the solids content) by this aftertreatment. It is, however, a serious disadvantage of this method that an additional step is required. A further disadvantage lies in that the reactions products of the alkylating agent with ammonia, an amino acid or an oligopeptide remain as impurities in the product.